In modern mechanical defibering of wood, pulp is screened under pressure to keep the quality of the accepted pulp, or accept, uniform. This may be carried out by controlling the amount of mass, i.e., the level of the mass surface, in the feeder or accept containers in the screening. Other alternatives include adjustments based on screening pressure and mass flow. In principle, these methods only control the capacity of the screening which is not, as such, in any way directly proportional to the quality of the screened pulp. Another way to control the screening such that the quality of the accepted pulp is also maintained as uniform as possible, irrespective of capacity variations, is based on the values of the flow-to-reject ratio and the feed consistency of the pulp supplied to the screening.
Although the adjustments used in prior art process control methods may be applied in standard or normal operating conditions, they cannot be used for controlling the process in exceptional circumstances, for example when refiners or grinding machines (referred to herein generically as “defibrators”) are switched on or off. Consequently, since in some cases a facility may comprise several defibrators, the quality of pulp can vary significantly, thereby affecting the further processes and the quality of the fibrous web made from the pulp.